1. Field of the Invention
The present invention relates to a cryoablation catheter, and more particularly to a cryoablation catheter for ablating heart tissue.
2. Description of the Prior Art
Certain medical procedures are performed using minimally invasive surgical techniques wherein one or more slender implements are inserted through one or more small incisions into a patient""s body. In the case of an ablation procedure, the surgical implement may include a rigid or flexible structure, or catheter, having an ablation device at or near its distal end that is placed adjacent to the tissue to be ablated. Radio frequency energy, microwave energy, laser energy, extreme heat, or extreme cold may be provided by the ablation device to destroy the tissue.
In certain cardiac procedures, such as the treatment of cardiac arrhythmias, selective ablation of cardiac tissue may be used to eliminate the source of the arrhythmia. One minimally invasive procedure, i.e., radio frequency (RF) catheter ablation, includes a preliminary step of conventional electrophysiology mapping followed by the creation of one or more ablated regions (lesions) in cardiac tissue using RF energy. Multiple lesions are frequently required because the effectiveness of each of the lesion sites cannot be predetermined with exactness due to limitations of conventional mapping. Often, five lesions, and sometimes as many as twenty lesions may be required before a successful result is attained.
Some deficiencies of radio-frequency ablation devices and techniques have been overcome by ice mapping prior to creating lesions, as taught by U.S. Pat. Nos. 5,423,807; 5,281,213 and 5,281,215. However, even though combined cryogenic mapping and ablation devices permit greater certainty and less tissue damage than RF devices and techniques, both the cryogenic and the RF devices are configured for spot or roughly circular tissue ablation.
Spot tissue ablation is acceptable for certain procedures. However, other procedures can be more therapeutically effective if multiple spot lesions are made along a predetermined line, or linear lesion, is created in a single ablative step. Radio-frequency ablation devices are known to be able to create linear lesions by dragging the ablation tip along a line while the ablation electrode is energized.
One problem associated with presently existing cryoablation catheters is that the outside dimensions of the cooling chamber, which is disposed at the distal tip of the catheter, must be limited by the size of the vessel. In other words, the outside diameter of the cooling chamber must be slightly smaller than the inside diameter of the vessel in order to permit passage of the cooling chamber through the vessel. Such small cooling chambers are relatively inefficient. It would be far better to have a cryoablation catheter with a large cooling, or gas expansion chamber, in order to increase the cooling efficiency of the device.
In the past it has been thought to be desirable to have a thick-walled cooling chamber, or dome, positioned at the distal tip of a catheter to serve both as an expansion chamber and a xe2x80x9cheat sinkxe2x80x9d to remove heat in order to cool adjacent tissue. Such thick-walled chambers present two problems with respect to efficient cooling and ablation. One problem is that with a thick-walled chamber having an outside diameter small enough to pass through the vasculature of the human body, the volume of the internal expansion chamber is very small. With a small volume cooling chamber, the cooling efficiency is very poor. In addition, the time required to cool such a thick-walled device is much greater than is required for a very thin-walled device. In other words, such prior devices have incorporated thick-walled cooling chambers in order to take advantage of the xe2x80x9cheat sinkxe2x80x9d effect, but with a significant loss in cooling efficiency.
The present invention provides a cryogenic ablation system with significantly improved cooling efficiency and reduction in cooling time.
In accordance with one aspect of the present invention there is provided a cryoablation catheter which includes an elongated flexible outer tubular member having a lumen extending therethrough, an inner tubular member extending through the lumen of the elongated flexible outer tubular member, and a generally cylindrical fluid expansion cooling chamber disposed on the distal end of the outer tubular member. The cooling chamber is in fluid engagement with the lumen of the outer tubular member. The cooling chamber also has an inside diameter substantially greater than the inside diameter of the outer tubular member and an outside diameter substantially equal to the outside diameter of the outer tubular member. The inner tubular member includes a proximal end adapted to be coupled to a source of cryoablation fluid, and also includes a fluid expansion nozzle disposed on its distal end which extends into the cooling chamber. Cooling fluid is applied to the proximal end of the inner tubular member and the fluid is caused to expand at the nozzle to thereby cause cooling to occur within the cooling chamber.
In accordance with another aspect of the present invention, the fluid expansion cooling chamber generally takes the form of a hemisphere, or dome, having an inside diameter substantially greater than the inside diameter of the outer tubular member and an outside diameter substantially equal to the outside diameter of the outer tubular member.
In accordance with still another aspect of the present invention, the outer wall of the cooling chamber is significantly thinner than the thickness of the outer tubular member, and is preferably approximately one-half of the thickness of the outer tubular member. More particularly, the cooling chamber preferably includes side walls having a thickness of about 0.10 inches. In another embodiment of the present invention, the fluid expansion nozzle takes the form of a Jewel-Thompson fluid expansion nozzle. The gas used for expansion and cooling preferably takes the form of nitrous oxide.
In accordance with still another embodiment of the present invention, the cryoablation catheter includes one or more mapping electrodes mounted on the distal end of the outer tubular member in order to provide a mapping function used in conjunction with the ablation procedure.
These and other objects of the present invention will be understood from the description of a preferred embodiment of the invention which follows.